Gamma Decay of the 154Sm Isovector Giant Dipole Resonance: Smekal-Raman Scattering as a Novel Probe of Nuclear Ground-State Deformation

The γ decays of the isovector giant dipole resonance (IVGDR) of the deformed nucleus Sm154 were measured using 21+-Smekal-Raman and elastic scattering of linearly polarized, quasimonochromatic photon beams. The two scattering processes were disentangled through their distinct angular distributions. Their branching ratio and cross sections were determined at six excitation energies covering the Sm154 IVGDR. Both agree with the predictions of the geometrical model for the IVGDR and confirm γ decay as an observable sensitive to the structure of the resonance. Consequently, the data place strong constraints on the nuclear shape, including the degree of triaxiality. The derived Sm154 shape parameters β=0.2925(25) and γ=5.0(15)° agree well with other measurements and recent Monte Carlo shell-model calculations.